Optical drives usually are equipped with a laser which generates a laser beam that is focused on the optical disc by at least one lens. The lens can be moved in radial and vertical (focus) directions by or together with an actuator. These movements caused by the actuator are only small. To make it possible to cover the whole relevant area of the optical disc with the focused laser beam, the laser as well as the actuator are part of a so called optical pick up unit which is mounted on a sledge that is moveable in radial directions. The focused laser beam is reflected by the optical disc and reaches a photo detector. The photo detector usually consists of several photo diodes that generate electrical signals on the basis of the amount of light which they receive. These electrical signals are processed and filtered in various ways. It is known to feed output signals into (among others) two PLL units and into a servo system.
PLL stands for Phase Lock Loop and such a PLL unit is used to try to determine the frequency of an incoming signal. To determine the frequency of the incoming signal within a certain frequency range, the PLL unit tries to lock on to the incoming signal by a lock on process. At the end of this lock on process the PLL unit can inform whether it was able to lock on to the incoming signal, and, if yes, the frequency of the incoming signal. In principle the lock on process can be executed in two ways. With the so called auto-lock process the PLL unit performs the lock on process autonomously. With the so called preset process the target or presetting frequency is provided to the PLL unit, wherein the presetting frequency is typically in the middle of the expected frequency range. The presetting process is usually faster than the auto-lock process.
For example in connection with Blue Ray Discs, the task of one PLL unit can be to detect the frequency of the HF signal (if present), and the task of the other PLL unit can be to detect the frequency of a wobble signal associated to a wobbled pre-groove.
The servo system is used to let the focused laser beam follow the track. This is realized by moving the lens by or with the actuator in radial and vertical directions and by moving the sledge carrying the actuator. The movement of the sledge can be monitored such that the actual position of the sledge is known during operation. However, as regards the actual position of the lens, the position information available for the sledge is not very accurate. This is particularly due to the fact that the motor assigned to the sledge works in discrete steps and that the actuator does also influence the position of the lens.
It is the object of the present invention to provide an optical drive and a method capable to accurately determine the reading and/or writing position, particularly—but without being limited thereto—when a Blue Ray Disc (BD) is used.